In the past, considerable attention has been given to the use of hydrogen as a fuel or fuel supplement. While the world's oil reserves are being rapidly depleted, the supply of hydrogen remains virtually unlimited. Hydrogen can be produced from coal, natural gas and other hydrocarbons, or formed by the electrolysis of water. Moreover hydrogen can be produced without the use of fossil fuels, such as by the electrolysis of water using nuclear or solar energy. Furthermore, hydrogen, although presently more expensive than petroleum, is a relatively low cost fuel. Hydrogen has the highest density of energy per unit weight of any chemical fuel and is essentially non-polluting since the main by-product of burning hydrogen is water.
While hydrogen has wide potential as a fuel, a major drawback in its utilization, especially in mobile uses such as the powering of vehicles, has been the lack of acceptable lightweight hydrogen storage medium. Conventionally, hydrogen has been stored in a pressure-resistant vessel under a high pressure or stored as a cryogenic liquid, being cooled to an extremely low temperature. Storage of hydrogen as a compressed gas involves the use of large and heavy vessels. In a steel vessel or tank of common design only about 1% of the total weight is comprised of hydrogen gas when it is stored in the tank at a typical pressure of 136 atmospheres. In order to obtain equivalent amounts of energy, a container of hydrogen gas would weigh about thirty times the weight of a container of gasoline. Additionally, transfer of a large sized vessel is very difficult. Furthermore, storage as a liquid presents a serious safety problem when used as a fuel for motor vehicles since hydrogen is extremely flammable. Liquid hydrogen also must be kept extremely cold, below −253° C., and is highly volatile if spilled. Moreover, liquid hydrogen is expensive to produce and the energy necessary for the liquefaction process is a major fraction of the energy that can be generated by burning the hydrogen.
Certain materials and alloys in solid state have the ability to absorb and desorb hydrogen. In this regard, they have been considered as a possible form of hydrogen-storage, due to their large hydrogen-storage capacity. Storage of hydrogen as a solid hydride can provide a greater volumetric storage density than storage as a compressed gas or a liquid in pressure tanks. Also, hydrogen storage in a solid hydride presents fewer safety problems than those caused by hydrogen stored in containers as a gas or a liquid. Solid-phase storage of hydrogen in a metal or alloy system works by absorbing hydrogen through the formation of a metal hydride under a specific temperature/pressure or electrochemical conditions, and releasing hydrogen by changing these conditions. Metal hydride systems have the advantage of high-density hydrogen-storage for long periods of time, since they are formed by the insertion of hydrogen atoms into the crystal lattice of a metal, alloy, or phase of the alloy. A desirable hydrogen storage material must have a high storage capacity relative to the weight of the material, a suitable desorption temperature/pressure, good absorption/desorption kinetics, good reversibility, no hysteresis, possess resistance to poisoning by contaminants including those present in the hydrogen gas, and be of a relatively low cost. If the material fails to possess any one of these characteristics it will not be acceptable for wide scale commercial utilization.
The hydrogen storage capacity per unit weight of material is an important consideration in many applications, particularly where the hydride does not remain stationary. In automotive applications, a material having a low hydrogen storage capacity relative to the weight of the material reduces the mileage, and hence the range of a hydrogen fueled vehicle utilizing such materials. A low hydrogen desorption temperature is desirable to reduce the amount of energy required to release the hydrogen from the material, as spending a great deal of energy to desorb hydrogen reduces the efficiency of the system. Furthermore, materials having a relatively low hydrogen desorption temperature are necessary for efficient utilization of the available exhaust heat from vehicles, machinery, fuel cells, or other similar equipment.
Good reversibility is needed to enable the hydrogen storage material to be capable of repeated absorption-desorption cycles without significant loss of its hydrogen storage capabilities. Good absorption/desorption kinetics are necessary to enable hydrogen to be absorbed or desorbed in a relatively short period of time. Resistance to contaminants, which the material may be subjected to during manufacturing and utilization, is required to prevent a degradation of acceptable performance. For example, magnesium hydrides have relatively high hydrogen storage capacities (up to 7.6 weight percent). However, te kinetics of the magnesium hydrides are poor in that only less than 1.0 weight percent is capable of desorption from the hydride at room temperature. Even at higher temperatures it is difficult to desorb all of the hydrogen stored in hydride form. Therefore, it is necessary to find a material or family of materials that will store more hydrogen with good reversibility and improved absorption/desorption kinetics.
A family of complex aluminum hydrides such as Na(AlH4), Li(AlH4), Zr(AlH4) and Mg(AlH4) have good theoretical reversible capacities between 4 weight percent and 8 weight percent as illustrated in Table 1 below. This family of complex aluminum hydrides are generally referred to as Alanates.
TABLE 1Theoretical Reversible CapacityNa(AlH4)5.6 weight percentLi(AlH4)7.9 weight percentZr(AlH4)3.9 weight percentMG(AlH4)7.0 weight percent
A reaction for the release of hydrogen from sodium alanate is as follows:NaAlH4<->⅓Na3AlH6+⅔Al+H2<->NaH+Al+ 3/2H2
In practice, the reversibility of the alanates could not be achieved until recently when it was found that the addition of a small amount of titanium catalyst made them reversible under certain conditions. However, doping with Ti has reduced the hydrogen storage capacities from 5.6 weight percent to 4 weight percent. While having low hydrogen storage capacities, titanium doped alanates offer good equilibrium thermodynamics and also very good packing densities with moderate volume expansion.
The process of introducing Ti catalyst may also be very complex. One process for doping sodium alanate with titanium is based on the removal of some of the sodium from the sodium alanate by reacting it with TiCl3. The titanium trichloride reacts with sodium to form sodium chloride under some strictly controlled conditions and in the process Ti catalyst is believed to enter the lattice. Although reversibility may now be promoted in alanates via the introduction of a titanium catalyst, the hydrogen storage capacity of the alanates are adversely affected and the process of including the titanium catalyst may be very complex.
Hydrogen storage is a technology critical to a wide variety of applications, some of the most prevalent being fuel cells, portable power generation, and hydrogen combustion engines. Such applications would benefit substantially from hydrogen storage materials capable of absorbing and desorbing higher amounts of hydrogen as compared to present day commercially available hydrogen storage materials. Hydrogen storage materials having increased absorption and desorption characteristics as compared to present day hydrogen storage materials will benefit such applications by providing longer operating life and/or range on a single charge for hydrogen power generators, fuel cells, and hydrogen internal combustion engines.